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The present invention relates to radiating systems and, more particularly, to improved radiation synthesizer systems enabling efficient use of small high-Q antennas by active control of energy transfer back and forth between an antenna reactance and a storage reactance.
The theory and implementation of Synthesizer Radiating Systems and Methods are described in U.S. Pat. No. 5,402,133 of that title as issued to the present inventor on Mar. 28, 1995. Further aspects are described in U.S. Pat. No. 6,229,494, titled Radiation Synthesizer Systems and Methods, as issued to the present inventor on May 8, 2001. These patents (xe2x80x9cthe ""133 patentxe2x80x9d and xe2x80x9cthe ""494 patentxe2x80x9d) are hereby incorporated by reference.
A basic radiation synthesizer circuit, as described in the ""133 patent, which combines transfer circuits in both directions using two switches is shown in FIG. 1a. This circuit functions as an active loop antenna where the loop antenna L is the high Q inductive load and a capacitor C is used as the storage reactor. The FIG. 1a circuit uses two RF type switching transistors, shown as switches RC and DC, for rate and direction control, respectively. Because the devices are operated in a switch mode, efficient operation is obtained since, in theory, no instantaneous power is ever dissipated by such devices. A slower switching device, shown as power control switch PC, can be used to add energy to the circuit from the power supply as energy is radiated. The voltage and current sensor terminals VS and CS, respectively, are used to monitor and calculate the total amount of stored energy at any instant in time, while a feedback control circuit is used to maintain the total energy at a preset value through use of the power control switch PC.
In the FIG. 1a circuit, when the direction control switch is open, energy can be transferred from current through the inductor L to voltage across the capacitor C, as illustrated by the L to C energy transfer diagram of FIG. 1b. With the rate control switch closed, current flows from ground, through diode D1 and L, and back to ground through the rate control switch RC. In the absence of circuit losses, the current would continue to flow indefinitely. When the rate control switch RC is opened, the inductor current, which must remain continuous, flows through diode D2 and charges up the capacitor C. The rate at which C charges up is determined by the switch open duty cycle of the switch RC. The capacitor will charge up at the maximum rate when the switch is continuously open. The charging time constant is directly proportional to the switch open duty cycle of the rate control switch RC.
When the direction control switch DC of FIG. 1a is closed, energy can be transferred from voltage across the capacitor to current through the inductor, as shown in the C to L energy transfer diagram of FIG. 1c. Diode D1 is always back biased and is, therefore, out of the circuit. When the rate control switch RC is closed, the capacitor C will discharge through L, gradually building up the current through L. If the rate control switch is opened, the capacitor will maintain its voltage while the inductor current flows in a loop through diode D2. In this C to L direction transfer mode, the rate is controlled by the switch closure duty cycle of switch RC. The maximum rate of energy transfer occurs when the switch RC is continuously closed. Its operation is the inverse of that in the other direction transfer mode (L to C).
It should be noted that, in either direction, charge or discharge is exponential. Therefore, the rate of voltage or current rise is not constant for a given rate control duty cycle. In order to maintain a constant rate of charging (ramp in voltage or current), it is necessary to appropriately modulate the duty cycle as charging progresses. Duty cycle determinations and other aspects of operation and control of radiation synthesizer systems are discussed at length in the ""133 patent (in which FIGS. 1a, 1b and 1c referred to above appear as FIGS. 8a, 8b and 8c).
In theory, since the power which is not radiated is transferred back and forth rather than being dissipated, lossless operation is possible. However, as recognized in the ""133 patent losses are relevant in high frequency switching operations, particularly as a result of the practical presence of ON resistance of switch devices and inherent capacitance associated with switch control terminals. While such device properties are associated with very small losses of stored energy each time a switch is closed, aggregate losses can become significant as high switching frequencies are employed. In addition, if small loop antennas are to be employed, for example, antenna impedance may be higher than basic switching circuit impedance levels, necessitating use of impedance matching circuits which may have less than optimum operating characteristics.
The basic radiation synthesizer circuit discussed above can be reduced to the simplified ideal model shown in FIG. 2. This model replaces the diodes in the basic circuit by ideal switches, and provides push-pull operation (current can flow in either direction through the loop antenna). The push-pull, or bipolar circuit, is more efficient than the single-ended circuit by a factor of 2 (3 dB). The FIG. 2 system includes four power switch devices comprising a switching circuit pursuant to the invention, a complete implementation of which is provided in the ""494 patent (see FIG. 12). The FIG. 2 system includes loop antenna 12, storage capacitor 14 and power switch devices 21, 22, 23 and 24, which will also be referred to as switch devices S1, S2, S3 and S4, respectively. Three possible states exist: linear charging of inductor current, linear discharging, and constant current. It is possible to synthesize any waveform using this circuit, with waveform fidelity dependent on sampling speed.
FIG. 2 shows a basic form of synthesizer radiating system. It uses a single switching circuit that is connected to the two input terminals of a standard loop antenna. Each switch may consist of several individual devices either connected in series or parallel in order to realize optimized performance at the desired radiation power level. At some frequencies of operation additional practical constraints may require consideration. As a first consideration, the device parameters may necessitate very low antenna input terminal impedance in order to realize acceptable performance. That impedance may not be compatible with a single-turn loop of appropriate size. As a second consideration, a single-turn loop may be subject to an electrical resonance when the antenna is moderately small. This resonance occurs when the distance around the loop perimeter approaches one-half wavelength at an operating frequency.
Pursuant to the ""494 patent, a multi-segment loop configuration using distributed switching electronics provides a solution addressing these considerations. An embodiment in which the antenna has been broken into four loop segments and uses four switching circuits controlled by synchronous signals is described by way of example in this patent. The effective terminal impedance that is presented to each sub-circuit is equal to 1/N times the total loop impedance where N is the number of loop segments. Hence, the optimum low-impedance antenna impedance level may be achieved by dividing the loop into the appropriate number of segments. The electrical resonance of this approach occurs when each segment length approaches one-half wavelength. Therefore, the resonance is increased in frequency by a factor of N over the non-segmented approach. It is possible, using this approach, to obtain acceptable performance at any frequency by properly segmenting the loop.
FIG. 3 shows a synthesizer radiating system 30, as described in the ""494 patent, employing a multi-segment loop radiator in the form of a single-turn loop separated into four segments 31-34. In FIG. 3, the single switching circuit of FIG. 2 is replaced by four switching circuits (i.e., for four xe2x80x9csub-circuitsxe2x80x9d) 10a, 10b, 10c, 10d, each of which is coupled to the ends of two successive ones of loop segments 31-34, as shown. Each of the sub-circuits 10a-d may be similar to switching circuit 10 of FIG. 2, except for the described coupling to loop segments 31-34 instead of to the ends of continuous loop 12 as in FIG. 2. The multi-segment loop radiator system 30 thus comprises a loop antenna element configured as a plurality of successive loop segments 31-34 and a like plurality of switching circuits 10a-d each coupled to a different pair of loop segments. Each switching circuit (i.e., sub-circuit) includes switch devices arranged for controlled activation as described above to transfer energy back and forth from the loop segments to which it is coupled to a portion of said storage capacitance (i.e., to one of capacitors 14a-d of FIG. 3). Although any number of segments may be utilized pursuant to design considerations as discussed, in FIG. 3 the plurality of successive loop segments consists of four loop segments 31-34, which are employed with a like plurality of switching circuits consisting of four switching circuits 10a-d, each having a respective capacitor 14a-d coupled thereto. Thus, in FIG. 3, the basic storage capacitance comprises a plurality of capacitive devices, one coupled to each switching circuit. Operational and other aspects of the FIG. 3 system are described in greater detail in the ""494 patent (in which FIG. 3 referred to above appears as FIG. 11).
With advances in wireless technology there is a steady progression toward the potential for implementation of a huge network in which human beings may effectively represent nodes in the network. There are many applications, especially in the area of military communications where the cellular model, with a central node through which all communications pass, is not most effective. Such non-cellular networks require the use of ground-to-ground communication. The propagation in ground-to-ground links is superior when the operating carrier frequency for the link is chosen in the lower regions of the frequency spectrum because of the relative immunity to blockage degradations that plague high frequency systems.
Although such propagation is superior at lower operating frequencies, operation at these frequencies has been typically avoided in the past because of the cumbersome antennas required for efficient coupling of transmit energy to radiation. Antennas are typically sized as an appreciable fraction of the wavelength at the operating frequency. At lower frequencies the wavelength may be in the 10 to 100 meter range, effectively limiting the portability of prior antennas.
Modern communication systems are typically wideband and frequency agile in order to suppress interference, and also to provide degrees of covertness or privacy. Thus, wideband antenna operation is desired. While in the past, it has been possible to reduce antenna size while maintaining efficiency over a very narrow band of frequencies, wideband efficiency has mandated the use of larger antennas.
A network of the type described may use each node in a semi-continuous manner, utilizing nodes not only for communication to and from that particular node but also as a relay for pass-through of data. As a result, a radio would not be utilized in a sporadic push-to-talk mode where it might be possible to temporarily erect and orient an antenna at appropriate times. It would also be desirable to provide a radiating system in a body-borne implementation that does not impede the host from performing other normal day-to-day activities. Further, if possible there should be no visual signature of the system that enables the antenna to be identified from afar. These two objectives are particularly important in military scenarios where a soldier will need to participate in normal combat operations while functioning as an active node in such a communication network. The combat operational environment also leads to the desirability that the system and antenna function equally well in any orientation, without degrading communication range. That contrasts to use of a typical wire stub antenna that must operate in a vertical orientation.
The electrical properties of the human body are far different from the open air that normally surrounds a radiating antenna. A body-borne antenna may typically be in close contact with human flesh. Because there are variations between different bodies, or even the same body from time to time, and there are variations that result from the presence of other equipment carried by that body, it is desirable that the antenna performance exhibit low sensitivity to such variations and characteristics. It would be undesirable to xe2x80x9cretunexe2x80x9d any antenna to the particular individual, or, in an extreme case, to require retuning for different clothing, how much perspiration is present, the presence of other equipment, or the position (standing, sitting, etc.) of a person using a body-borne antenna.
Objects of the invention are, therefore, to provide new and improved radiating systems, including crossed-loop and synthesizer radiating systems, providing one or more of the following advantages or characteristics:
suitability for body-borne use;
conformal to body;
absence of protrusions from body creating visual signature;
operability not dependent on body orientation;
wide instantaneous bandwidth operation;
high efficiency signal radiation;
electrically small antenna elements;
operability not limited by body effects or nearby objects; and
absence of hazardous electromagnetic field effects.
In accordance with the invention, a crossed-loop radiation synthesizer system, wherein energy is transferred back and forth between each loop and storage capacitance via controlled activation of switch devices, includes a first loop antenna element configured as a plurality of successive loop segments and an offset loop antenna element configured as a plurality of successive loop segments and having an operating position offset in azimuth from the first loop antenna element. A first plurality of switch modules are each coupled to a different pair of loop segments of the first loop antenna element and a second plurality of switch modules are each coupled to a different pair of loop segments of the offset loop antenna element. Each switch module includes switch devices arranged for controlled activation to transfer energy back and forth between the storage capacitance and a loop antenna element. The radiating system may also include a coupler configuration to couple to the switch modules signals representative of signals to be transmitted, with signals coupled to the second plurality of switch modules having a phase offset (e.g., quadrature phase) relative to signals coupled to the first plurality of switch modules. The radiating system may further include an input/output unit (e.g., a radio) responsive to input information to provide signals representative of signals to be transmitted and also responsive to received signals to provide output signals representative of information contained in received signals. The radiating system as described may be combined with a wearable garment configured to support the loop antenna elements and switch modules, with the offset loop antenna element supported in an offset-in-azimuth operating position.
Radiation synthesizer systems may utilize optical modulators responsive to signals representative of signals to be transmitted and optical signal paths coupled between an optical modulator and switch modules. Operating power to the switch modules may be provided via antenna element loop segments which each include a plurality of parallel conductor portions arranged to enable coupling of a plurality of DC voltages to a switch module.
In a further embodiment, a crossed-loop radiation synthesizer system, wherein energy is transferred back and forth between each loop and storage capacitance via controlled activation of switch devices, may include a first loop antenna element and an offset loop antenna element having an operating position offset in azimuth from the first loop antenna element (e.g., in azimuth quadrature). The system includes at least one switch module coupled to the loop antenna elements, each switch module including switch devices arranged for controlled activation to transfer energy back and forth between the storage capacitance and a loop antenna element.
In an additional embodiment, a crossed-loop radiation system includes a first loop antenna element and an offset loop antenna element. A wearable garment is provided to support the loop antenna elements with the offset loop antenna element in an operating position offset in azimuth from the first loop antenna element. A coupler configuration is arranged to couple first signals to the first loop antenna element and second signals, comprising a phase offset replica of the first signals, to the offset loop antenna element.
For a better understanding of the invention, together with other and further objects, reference is made to the accompanying drawings and the scope of the invention will be pointed out in the accompanying claims.